Biomedical and analytical instrumentation professionals have recognized the benefits of using different wavelength excitations in Raman spectroscopy for many applications, such as lab analysis, patient monitoring, and field work. Since a Raman spectroscopic analyzer (or spectrograph) has a single wavelength excitation and spectral range, multiple analyzers covering different wavelength excitations have been used. For example, a Raman analyzer optimized for a wavelength excitation range from about 532 nm to about 640 nm has been used with a Raman 532 nm excitation laser and a Raman analyzer optimized for a wavelength excitation range from about 785 nm to about 1100 nm has been used with a Raman 785 nm excitation laser. It has been virtually impossible for the different excitation lasers to use the same analyzer. However, providing multiple analyzers has been problematic for a number of reasons. The analyzers are large and expensive, resulting in higher cost and space requirements for housing more than one analyzer. Moreover, the analyzers are fragile and complex, requiring highly trained persons to operate them.
Commonly used excitation wavelengths have been about 532 nm and about 785 nm. Recently, however, it has been determined that longer excitation wavelengths in the near infrared, e.g., about 1064 nm, can be advantageous in Raman analysis of highly fluorescent samples, especially biological samples, such as tissue or skin samples, measured both in-vivo and in-vitro. For example, it has been determined that longer excitation wavelengths can reduce or eliminate fluorescence interference with the Raman signal to be analyzed. This, of course, adds another wavelength excitation range for which a Raman analyzer would be needed. For example, a Raman 1064 nm excitation laser would need a Raman analyzer optimized for a wavelength excitation range from about 1064 nm to about 1700 nm.
Due in part to the above-mentioned problems associated with using multiple analyzers, some professionals have opted to use a single Raman analyzer that most effectively works for their particular application. However, selecting the analyzer has not been without concern. For example, along with the analyzer, the corresponding excitation laser must also be selected. It is well known among analytical chemists and vibrational spectroscopy professionals that, albeit Raman spectroscopy is a true “color-blinded” technology in term of excitation laser wavelengths vs. Raman shifts, special attention must be given when choosing an excitation laser. The laser wavelength (and power) must be selected in reference to the target sample(s) to be analyzed. Moreover, tradeoffs must be made regarding laser availability, Raman detection sensitivity, sample damage, and sample fluorescence interference.
Additional, the analysis technology used by the analyzer has not been without concern. Primarily Fourier transform (FT) technology has been used in the analyzers. However, such technology involves moving parts, e.g., a moving reflection mirror, large size elements, cumbersome operation, e.g., cryogenic cooling of photodetectors, and vibration or shock. One example of this technology is a Michaelson interferometer. As a result, this technology can cause instability in the Raman analysis.
To address some of the concerns associated with FT technology, a dispersive Raman analyzer based on a transmissive volume phase grating (VPG) has been developed, which can operate without moving parts. The analyzer is used in conjunction with an photodetector, such as a CCD array or an InGaAs array, operated at room temperature or cooler temperatures via thermal electrical (TE) cooling. Accordingly, the dispersive analyzer reduced or eliminated vibration or shock, moving parts, and some cumbersome operation, e.g., regarding cooling of photodetectors.
To address size and expense of Raman analyzers, optical telecommunications technology began to enter the spectroscopy area, particularly with regard to the excitation wavelength range of about 1000 nm to about 1700 nm. Such optical technology includes light source, detection devices, and so on, resulting in more reliable and less expensive spectroscopy instruments. Miniature lasers, compact narrow and broadband light sources, holographic and other optical elements, sensitive solid state optoelectronics, and fast computer chips are only a few examples of the contributions made by optical technology to spectroscopy.
While spectroscopy advances have addressed many concerns as described above, there remains the difficulty in providing different wavelength excitations for Raman spectroscopy applications in a productive, cost effective, efficient manner.